You'll have to pay extra council tax for her if you get single person CT discount.
She will have expenses and is earning. Will cost more in heating, electrics, better WiFi etc and food. So it's more than reasonable to charge living expenses and nominal rent.
If it's a gap year you will want her to save for uni next year too. At least £2-3k. Tell her to save 1/3 to half her wages and her rent / living costs charge will be less if she does.
For £200 week wages £900 month )+ you said she may pick up more wages and earn more, I would ask for £250 a month and she pays her own mobile phone bill and transport and saves 1/4-1/3 of her wages into savings account for uni.
If she saves 1/2 wages by Standing order, I would drop it to £150. But I would insist on being shown that. Because you live alone in bedroom in shared house without food and transport and mobile phone + insurance, would be £400-500 at least a month, so is bargain for her.
If you can set aside some of that "rent and living costs contribution" (make her aware it is still hugely discounted rate for family!) without telling her, to also make her a savings account, if you can afford it, depending on what you negotiate with her, then that would be a great idea. Then she'll love you for it. I made secret savings accounts for mine (but are going to uni at 18) and they were delighted.
I would need the money if my adult DCs came to live with me and was working earning not studying as I'd have extra costs, DS age 19 costs me extra £250 in food a month (he almost doubles our food bill & shoots up our electric island heating use when home
!) and it would cost me an extra £60 in council tax if he lived here permanently (as a working adult) rather than the 4-5 months temporary stay a year whilst at uni outside term time!!
I also subsidise him a fair bit of the time, when he runs out of food money away at Uni!
I sometimes do a £70 online shop delivery via Morrisons on Amazon to his uni house with basics (huge pasta and rice packs, meat, tins etc) at start of year /or term as I learnt that quickly! He can't carry much to the house from supermarket as it's such a long walk. Yeah he could do it himself but his budget isn't enough to do huge "shops", and loves that his mum does it adhoc for him.